The phrase "6 feet under" originated in London, England in 1665. It came about as London was being ravaged by the Beubonic Plague. The plague was so rampant that the death rate reached 7,000 per week at its height.
The mayor of London at the time issued a decree that all plague deaths had to be buried at least "6 feet under" to help halt the spread of infection. See, Londoners believed the plague could be spread by the dead as well as the living and 6 feet underground was deep enough to contain the infection.
Of course, people later learned that the plague was spread by fleas from rats. The great fire of London in 1666 that basically wiped out the rat population in the city and gave a reprieve from the plague. The city was saved from disease but the phrase "six feet under" stuck and is still widely used today.